Photoelectrochemical reaction system

ABSTRACT

A photoelectrochemical reaction system of an embodiment includes a CO 2  conversion part and a CO 2  reduction part. The CO 2  conversion part converts carbon dioxide into at least one intermediate substance selected from the group consisting of a metal carbonate and a metal hydrogen carbonate by an aqueous solution containing a metal hydroxide, and generates a reaction solution containing the intermediate substance. The CO 2  reduction part includes a one-liquid reaction vessel where the reaction solution containing the intermediate substance is led in, an oxidation electrode and a reduction electrode immersed in the reaction solution, and a photoelectric conversion element electrically connected to the oxidation and reduction electrodes. H 2 O is oxidized in a vicinity of the oxidation electrode to generate O 2 , and the intermediate substance is reduced in a vicinity of the reduction electrode to generate a carbon compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior International Application No. PCT/JP2015/001205 filed on Mar. 5, 2015, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-060061 filed on Mar. 24, 2014; the entire contents of all of which are incorporated herein by reference.

TECHNICAL FIELD

Embodiments disclosed herein generally relate to a photoelectrochemical reaction system.

BACKGROUND ART

From viewpoints of energy problems and environmental problems, a technology effectively reducing CO₂ by light energy such as plants has been required. The plants use a system which is excited in two stages by the light energy called as a Z scheme. Namely, the plants obtain electrons from water (H₂O) by the light energy, and synthesize cellulose and saccharide by reducing carbon dioxide (CO₂) using the electrons. As a device performing an artificial photosynthesis, development of a photoelectrochemical reaction device reducing (decomposing) CO₂ by the light energy has been advanced.

As the artificial photoelectrochemical reaction device, a device in a two-electrode system is known in which an electrode having a reduction catalyst reducing carbon dioxide (CO₂) and an electrode having an oxidation catalyst oxidizing water (H₂O) are included, and these electrodes are immersed in water where CO₂ is dissolved. When water is used as an electrolytic solution, a dissolved concentration of CO₂ is low, and therefore, it is impossible to increase a decomposition efficiency of CO₂. Further, it has been studied to use a stack in which a photoelectric conversion layer is held by a pair of electrodes as a photoelectrochemical reactor in which water (H₂O) is decomposed by the light energy to obtain oxygen (O₂) and hydrogen (H₂). As a CO₂ absorbent, an aqueous solution containing amine molecules, an ionic liquid, and so on have been studied. In addition to the photoelectrochemical reactor, an alkaline solution such as an aqueous potassium hydroxide solution and an aqueous sodium hydroxide solution, or an amine solution is used as the electrolytic solution in a conventional CO₂ reductor (CO₂ electrolytic device).

In a conventional photoelectrochemical reactor, the amine solution used as the CO₂ absorbent has low chemical stability, and it is gradually oxidized under a natural state. An oxidation electrode side of the photoelectrochemical reactor is in a strong oxidation environment, and therefore, amine molecules in the aqueous solution are preferentially oxidized, and a recovery and a reuse of the amine solution cannot be performed. In the conventional photoelectrochemical reactor, an inside of an electrolytic vessel is isolated into an oxidation electrode side and a reduction electrode side. However, this incurs complication of a cell structure, and therefore, a device cost increases, and the device is easy to be further large-sized. The ionic liquid is chemically stable, but it is expensive in itself, and therefore, the device cost increases. In the conventional CO₂ electrolytic device, a transport property and a transport efficiency of CO₂ from a device discharging CO₂ to the electrolytic device are not considered, and a configuration as a photoelectrochemical reaction system is not developed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration chart of a photoelectrochemical reaction system according to a first embodiment.

FIG. 2 is a view illustrating a first example of a photoelectrochemical module used for the photoelectrochemical reaction system illustrated in FIG. 1.

FIG. 3 is a view illustrating a second example of a photoelectrochemical module used for the photoelectrochemical reaction system illustrated in FIG. 1.

FIG. 4 is a view illustrating an oxidation electrode used for the photoelectrochemical module illustrated in FIG. 2.

FIG. 5 is a view illustrating a reduction electrode used for the photoelectrochemical module illustrated in FIG. 2.

FIG. 6 is a view illustrating a photoelectric conversion element used for the photoelectrochemical module illustrated in FIG. 2.

FIG. 7 is a view illustrating a concrete example of the photoelectric conversion element illustrated in FIG. 6.

FIG. 8 is a view explaining operations of the photoelectrochemical module illustrated in FIG. 2.

FIG. 9 is a view explaining operations of the photoelectrochemical module illustrated in FIG. 3.

FIG. 10 is a configuration chart of a photoelectrochemical reaction system according to a second embodiment.

DETAILED DESCRIPTION

According to one embodiment, there is provided a photoelectrochemical reaction system including: a conversion part converting carbon dioxide into at least one intermediate substance selected from the group consisting of a metal carbonate and a metal hydrogen carbonate by an aqueous solution containing a metal hydroxide, and generating a reaction solution containing the intermediate substance; a transfer part transferring the reaction solution containing the intermediate substance; and a reduction part including a one-liquid reaction vessel where the reaction solution is led in by the transfer part, an oxidation electrode immersed in the reaction solution to oxidize water; a reduction electrode immersed in the reaction solution to reduce the intermediate substance, and a photoelectric conversion element electrically connected to the oxidation and reduction electrodes and performing a charge separation by light energy.

Hereinafter, a photoelectrochemical reaction system of an embodiment is described with reference to the drawings.

First Embodiment

FIG. 1 is a view illustrating a configuration of a photoelectrochemical reaction system according to a first embodiment. A photoelectrochemical reaction system 100 of the first embodiment is provided subsidiary to a CO₂ generation part 100X which generates gas containing carbon dioxide (CO₂). The photoelectrochemical reaction system 100 includes a CO₂ conversion part 102, a reaction solution transfer part 103, a CO₂ reduction part 104, a reaction solution adjustment part 105, a reaction solution reflux part 106, a product collection part 107, and a reaction solution storage part 108. As a representative example of the CO₂ generation part 100X, a power station can be cited. The CO₂ generation part 100X is not limited thereto, and may be an ironwork, a chemical factory, a garbage incineration plant, or the like. The CO₂ generation part 100X is not particularly limited. The photoelectrochemical reaction system 100 according to the embodiment is able to enable reduction in size of the CO₂ reduction part 104 as described later, and therefore, it is effective for small-sized plants such as the garbage incineration plant without being limited to large-sized plants such as the power station and the ironwork.

The gas containing CO₂ generated at the CO₂ generation part 100X, for example, exhaust gas discharged from the power station, the ironwork, the chemical factory, the garbage incineration plant, or the like is transferred to the CO₂ conversion part 102 of the photoelectrochemical reaction system 100. The exhaust gas may be transferred to the CO₂ conversion part 102 after impurities such as sulfur oxide in the exhaust gas are removed depending on components and so on of the exhaust gas discharged from the CO₂ generation part 100X. The photoelectrochemical reaction system 100 may include an impurity removal part 101. The impurity removal part 101 is not limited to be arranged between the CO₂ generation part 100X and the CO₂ conversion part 102, and may be arranged everywhere in a circulation route of carbon dioxide. The impurity removal part 101 may be arranged between the CO₂ conversion part 102, the reaction solution transfer part 103, the CO₂ reduction part 104, the reaction solution reflux part 106, and the reaction solution storage part 108. The impurities is not limited to the component in the exhaust gas, and may be a decomposing material and a chemically changed material of a pipe and the reaction solution, a eluted material from a pipe and a tank by the reaction solution, metal ions from the CO₂ reduction part 104 and so on. As the impurity removal part 101, various dry-type or wet-type gas processing apparatuses, a ion-exchange resin absorbing metal ions, a filter removing sulfur oxides and nitrogen oxides, a filter removing a physical decomposing material of a pipe, a tank and a stirrer, and so on can be cited.

At the CO₂ conversion part 102, CO₂ is converted into at least one intermediate substance selected from a metal carbonate and a metal hydrogen carbonate. The CO₂ conversion part 102 includes a reaction tank where an aqueous solution containing a metal hydroxide which converts CO₂ into the intermediate substance is accommodated. In the reaction tank where the aqueous solution of the metal hydroxide is accommodated, gas containing CO₂ is injected from a gas supply pipe. CO₂ injected into the aqueous solution is converted into at least one intermediate substance selected from the metal carbonate and the metal hydrogen carbonate by the metal hydroxide. CO₂ is converted into the intermediate substance by the metal hydroxide, and thereby, a reaction solution (aqueous solution) containing the intermediate substance is generated in the reaction tank. Namely, at the CO₂ conversion part 102, the reaction solution (aqueous solution) containing at least one intermediate substance selected from the metal carbonate and the metal hydrogen carbonate and water (H₂O) is generated.

The metal hydroxide which converts CO₂ into the intermediate substance is preferably a hydroxide of at least one metal selected from an alkaline metal (group 1 element) and an alkaline earth metal (group 2 element). The metal hydroxide is more preferably the hydroxide of at least one metal selected from lithium (Li), sodium (Na), potassium (K), rubidium (Rb), beryllium (Be), magnesium (Mg), calcium (Ca), and strontium (Sr). A pH of the aqueous solution containing the metal hydroxide is preferably adjusted to a range of 7 to 14. To increase a reactivity between CO₂ and the metal hydroxide, it is preferable to adjust the pH of the aqueous solution containing the metal hydroxide into a strong alkaline region. On the other hand, to suppress corrosion of components of the CO₂ conversion part 102, the CO₂ reduction part 104, and so on, the pH of the aqueous solution containing the metal hydroxide is preferably adjusted to a weak alkaline region.

A case when sodium hydroxide (NaOH) is used as the metal hydroxide is described as an example. CO₂ injected into the reaction tank where the aqueous solution of the metal hydroxide is accommodated is converted into sodium carbonate and sodium hydrogen carbonate based on the following (1) expression and (2) expression.

NaOH+CO₂ - - - >NaHCO₃  (1)

2NaOH+CO₂ - - - >Na₂CO₃+H₂O  (2)

There is a case when sodium carbonate is further converted into sodium hydrogen carbonate based on the following (3) expression.

Na₂CO₃+CO₂+H₂O - - - >2NaHCO₃  (3)

The other alkaline metals (group 1 element) are almost the same.

When the hydroxide of the alkaline earth metal (group 2 element) such as calcium hydroxide (Ca(OH)₂) is used as the metal hydroxide, CO₂ is converted into calcium carbonate and calcium hydrogen carbonate based on the following (4) expression and (5) expression.

Ca(OH)₂+CO₂ - - - >CaCO₃+H₂O  (4)

CaCO₃+H₂O+CO₂ - - - >Ca(HCO₃)₂  (5)

The other alkaline earth metals (group 2 element) are almost the same.

The reaction solution (aqueous solution) containing the intermediate substance (the metal carbonate and the metal hydrogen carbonate) generated at the CO₂ conversion part 102 is transferred to the CO₂ reduction part 104 by the reaction solution transfer part 103. The CO₂ conversion part 102 and the CO₂ reduction part 104 are not necessarily operated simultaneously. For example, when the operation of the CO₂ reduction part 104 is stopped at night though the CO₂ conversion part 102 operates also at night, the reaction solution containing the intermediate substance generated at the CO₂ conversion part 102 is stored at the reaction solution storage part 108. The reaction solution stored at the reaction solution storage part 108 is transferred to the CO₂ reduction part 104 at the operation time of the CO₂ reduction part 104.

The CO₂ reduction part 104 includes a photoelectrochemical module 1 illustrated in FIG. 2 and FIG. 3. FIG. 2 illustrates a photoelectrochemical module 1A in which a photoelectric conversion element is disposed outside the reaction solution, and an oxidation electrode and a reduction electrode are immersed in the reaction solution. FIG. 3 illustrates a photoelectrochemical module 1B in which a stack (photoelectrochemical cell) of the oxidation electrode, a photoelectric conversion layer, and the reduction electrode is immersed in the reaction solution.

The photoelectrochemical module 1A (104) illustrated in FIG. 2 includes a one-liquid type reaction vessel 3 accommodating a reaction solution 2, an oxidation electrode 4 and a reduction electrode 5 immersed in the reaction solution 2, and a photoelectric conversion element 6 disposed at outside the reaction vessel 3 and electrically connected to the oxidation electrode 4 and the reduction electrode 5. A reaction solution lead-in pipe 7 a leading in the reaction solution 2 by the reaction solution transfer part 103, an adjustment solution lead-in pipe 7 b leading in an adjustment solution of the reaction solution 2 from the reaction solution adjustment part 105, a reaction solution lead-out pipe 7 c leading out a solution after a reaction by the reaction solution reflux part 106, a reaction solution discharge pipe 7 d discharging the solution after the reaction, and a product send-out pipe 7 e sending out a gaseous reaction product to the product collection part 107 are connected to the reaction vessel 3.

In the reaction vessel 3 of the photoelectrochemical module 1A, the reaction solution containing the intermediate substance generated at the CO₂ conversion part 102 is led in via the reaction solution lead-in pipe 7 a. A lead-in amount of the reaction solution is adjusted to generate a predetermined space S at an upper part of the reaction vessel 3. The gaseous product generated by an oxidation-reduction reaction in the reaction vessel 3 is collected at the upper space S of the reaction vessel 3, and thereafter, transferred to the product collection part 107 via the product send-out pipe 7 e. In the reaction vessel 3, the adjustment liquid is further led in according to need via the adjustment liquid lead-in pipe 7 b. The reaction solution in the reaction vessel 3 is adjusted by the adjustment liquid to have a desired concentration, characteristics, and so on. The reaction solution in which the oxidation-reduction reaction is performed in the reaction vessel 3 is refluxed to the CO₂ conversion part 102 via the reaction solution lead-out pipe 7 c. A part of the reaction solution after the reaction is discharged out of a system via the reaction solution discharge pipe 7 d according to need. The lead-in and lead-out of the reaction solution may be continuously performed, or may be performed discontinuously in a batch mode.

The reaction vessel 3 is preferably formed by a material which does not chemically react with the reaction solution 2, and difficult to be altered by the energy of sunlight. As such a material, for example, there can be cited resin materials such as a polyetheretherketone (PEEK) resin, a polyamide (PA) resin, a polyvinylidene fluoride (PVDF) resin, a polyacetal (POM) resin (copolymer), a polyphenyleneether (PPE) resin, an acrylonitrile butadiene styrene copolymer (ABS), a polypropylene (PP) resin, a polyethylene (PE) resin, and so on.

At the oxidation-reduction reaction time of the reaction solution 2, the reaction vessel 3 may include an agitator stirring the reaction solution 2 so that the reaction is uniformly and efficiently performed in the reaction vessel 3. The upper space S of the reaction vessel 3 is preferably a complete hermetical seal except the product send-out pipe 7 e to efficiently collect and discharge a gas product. The reaction solution 2 is preferably led in to be within a range of 50% to 90% relative to an internal capacity of the reaction vessel 3, and more preferably led in to be within a range of 70% to 90% of the reaction vessel 3 so as to keep the upper space S of the reaction vessel 3.

The reaction solution 2 led into the reaction vessel 3 is adjusted to have the concentration and characteristics suitable for an oxidation reaction of H₂O and a reduction reaction of the intermediate substance by the reaction solution adjustment part 105. Specifically, it is preferable to lead in water and the aqueous solution of the same metal hydroxide as the CO₂ conversion part 102 into the reaction vessel 3 via the adjustment liquid lead-in pipe 7 b so that the pH of the reaction solution 2 is within a range of 10.0 to 14.0. A redox couple may be added to the reaction solution 2 according to need. As the redox couple, for example, there can be cited Fe³⁺/Fe²⁺ and IO³⁻/I⁻.

In the reaction vessel 3 where the reaction solution 2 is led, the oxidation electrode 4 and the reduction electrode 5 are disposed to be immersed in the reaction solution 2. The oxidation electrode 4 includes, for example, a support substrate 4 a and oxidation catalyst layers 8 formed at both surfaces thereof as illustrated in FIG. 4. The support substrate 4 a of the oxidation electrode 4 is formed by a material having conductivity. As forming materials of the support substrate 4 a, there can be cited metals such as Cu, Al, Ti, Ni, Fe, Ag, an alloy containing at least one of these metals, a conductive resin, and so on. For example, a metal plate and an alloy plate are used for the support substrate 4 a in consideration of formability of the oxidation catalyst layer 8. The support substrate 4 a may be constituted by a porous body of the metal, the alloy, and the conductive resin.

The oxidation catalyst layer 8 has functions receiving positive holes from the support substrate 4 a of the oxidation electrode 4, reacting with H₂O in the reaction solution 2, and oxidizing H₂O. A composing material of the oxidation catalyst layer 8 preferably contains an oxide or a hydroxide of at least one metal selected from Fe, Ni, Co, Cu, Ti, V, Mn, Ru, and Ir. As concrete composing materials of the oxidation catalyst layer 8, there can be cited one or two or more composite materials selected from RuO₂, NiO, Ni(OH)₂, NiOOH, Co₃O₄, Co(OH)₂, CoOOH, FeO, Fe₂O₃, MnO₂, Mn₃O₄, Rh₂O₃, and IrO₂. The oxidation catalyst layer 8 is to accelerate the oxidation reaction of H₂O at the oxidation electrode 4, and therefore, it is possible not to have the oxidation catalyst layer 8 when a reaction rate of the oxidation reaction by the support substrate 4 a of the oxidation electrode 4 is enough.

The reduction electrode 5 includes a support substrate 5 a and reduction catalyst layers 9 formed at both surfaces thereof as illustrated in FIG. 5. The support substrate 5 a of the reduction electrode 5 is formed by a material having conductivity. As forming materials of the support substrate 5 a, there can be cited metals such as Cu, Al, Ti, Ni, Fe, Ag, an alloy containing at least one of these metals, a conductive resin, and so on. For example, a metal plate and an alloy plate are used for the support substrate 5 a in consideration of formability of the reduction catalyst layer 9. The support substrate 5 a may be constituted by a porous body of the metal, the alloy, the conductive resin, or the like.

The reduction catalyst layer 9 has functions receiving electrons from the support substrate 5 a of the reduction electrode 5, and reducing the intermediate substance in the reaction solution 2, namely, the metal carbonate and the metal hydrogen carbonate, and CO₂ generated by these. A composing material of the reduction catalyst layer 9 preferably contains metals such as Au, Ag, Zn, Cu, Hg, Cd, Pb, Ti, In, Sn, metal complexes such as a ruthenium complex, a rhenium complex, carbon materials such as graphene, CNT (carbon nanotube), fullerene, ketjen black, and so on. The reduction catalyst layer 9 is to accelerate the reduction reaction of CO₂ at the reduction electrode 5, and therefore, it is possible not to have the reduction catalyst layer 9 when a reaction rate of the reduction reaction by the support substrate 5 a of the reduction electrode 5 is enough.

The photoelectric conversion element 6 is electrically connected to the oxidation electrode 4 and the reduction electrode 5, and thereby, electrons and positive holes are exchanged between the oxidation electrode 4 and the reduction electrode 5. The photoelectric conversion element 6 performs a charge separation by the light energy. As the photoelectric conversion element 6, there can be cited a pin junction, a pn junction, an amorphous silicon solar cell, a multijunction solar cell, a single crystal silicon solar cell, a polycrystalline silicon solar cell, a dye sensitized solar cell, an organic thin-film solar cell, and so on. It is necessary for the photoelectric conversion element 6 to create a potential difference which is higher than a difference between a standard oxidation-reduction potential of the oxidation reaction of H₂O generated in the vicinity of the oxidation electrode 4 and a standard oxidation-reduction potential of the reduction reaction of CO₂ generated in the vicinity of the reduction electrode 5. Namely, the photoelectric conversion element 6 is one capable of providing the energy necessary for simultaneously generating the oxidation reaction of H₂O and the reduction reaction of CO₂.

The photoelectric conversion element 6 is made up of a first electrode layer 11, a photoelectric conversion layer (photovoltaic layer) 31, and a second electrode layer 21 as illustrated in FIG. 6. FIG. 7 illustrates a concrete example of the photoelectric conversion element 6 using the silicon solar cell (pin junction) as the photoelectric conversion layer 31. The second electrode layer 21 is formed by metals such as Cu, Al, Ti, Ni, Fe, Ag, an alloy such as SUS which contains at least one of these metals, a conductive resin, semiconductors such as Si and Ge, and so on. A metal plate, an alloy plate, a resin plate, or a semiconductor substrate is used for the second electrode layer 21.

The photoelectric conversion layer 31 is formed on the second electrode layer 21. The photoelectric conversion layer 31 is made up of a reflection layer 32, a first photoelectric conversion layer 33, a second photoelectric conversion layer 34, and a third photoelectric conversion layer 35. The reflection layer 32 is formed on the second electrode layer 21, and includes a first reflection layer 32 a and a second reflection layer 32 b formed in sequence from a lower part side. Metals such as Ag, Au, Al, Cu, an alloy containing at least one of these metals, and so on are used for the first reflection layer 32 a having light reflectivity and conductivity. The second reflection layer 32 b is provided to enhance the light reflectivity by adjusting an optical distance. The second reflection layer 32 b is joined to an n-type semiconductor layer of the later-described photoelectric conversion layer 31, and therefore, it is preferably formed by a material having a light transmission property and in which an ohmic contact with the n-type semiconductor layer is possible. Transparent conductive oxides such as ITO (indium tin oxide), zinc oxide (ZnO), FTO (fluorine-doped tin oxide), AZO (aluminum-doped zinc oxide), ATO (antimony-doped tin oxide) are used for the second reflection layer 32 b.

The first photoelectric conversion layer 33, the second photoelectric conversion layer 34, and the third photoelectric conversion layer 35 are solar cells each using a pin junction semiconductor, and light absorption wavelengths thereof are different. These are stacked in a planar state, and thereby, it is possible to absorb the light in a wide wavelength of the sunlight by the photoelectric conversion layer 31, and to efficiently use the light energy of the sunlight. The photoelectric conversion layers 33, 34, 35 are connected in series, and therefore, it is possible to obtain a high open-circuit voltage.

The first photoelectric conversion layer 33 is formed on the reflection layer 32, and includes an n-type amorphous silicon (a-Si) layer 33 a, an intrinsic amorphous silicon germanium (a-SiGe) layer 33 b, and a p-type microcrystal silicon (mc-Si) layer 33 c formed in sequence from a lower part side. The a-SiGe layer 33 b is a layer absorbing light in a long wavelength region at approximately 700 nm. At the first photoelectric conversion layer 33, the charge separation occurs by the light energy in the long wavelength region.

The second photoelectric conversion layer 34 is formed on the first photoelectric conversion layer 33, and includes an n-type a-Si layer 34 a, an intrinsic a-SiGe layer 34 b, and a p-type mc-Si layer 34 c formed in sequence from a lower part side. The a-SiGe layer 34 b is a layer absorbing light in an intermediate wavelength region at approximately 600 nm. At the second photoelectric conversion layer 34, the charge separation occurs by the light energy in the intermediate wavelength region.

The third photoelectric conversion layer 35 is formed on the second photoelectric conversion layer 34, and includes an n-type a-Si layer 35 a, an intrinsic a-Si layer 35 b, and a p-type mc-Si layer 35 c formed in sequence from a lower part side. The a-Si layer 35 b is a layer absorbing light in a short wavelength region at approximately 400 nm. At the third photoelectric conversion layer 35, the charge separation occurs by the light energy in the short wavelength region.

The first electrode layer 11 is formed on the p-type semiconductor layer (the p-type mc-Si layer 35 c) of the photoelectric conversion layer 31. The first electrode layer 11 is preferably formed by a material in which the ohmic contact with the p-type semiconductor layer is possible. Metals such as Ag, Au, Al, Cu, an alloy containing at least one of these metals, transparent conductive oxides such as ITO, ZnO, FTO, AZO, ATO, and so on are used for the first electrode layer 11. The first electrode layer 11 may have, for example, a structure in which the metal and the transparent conductive oxide are stacked, a structure in which the metal and the other conductive materials are compound, a structure in which the transparent conductive oxide and the other conductive materials are compound, and so on.

At the photoelectric conversion element 6 illustrated in FIG. 7, irradiated light passes through the first electrode layer 11 to reach the photoelectric conversion layer 31. The first electrode layer 11 has light transmission property for the irradiated light. At the photoelectric conversion layer 31, the charge separation occurs by the light energy in each wavelength region of the irradiated light (sunlight and so on). At the photoelectric conversion element 6 illustrated in FIG. 7, the positive holes are separated toward the first electrode layer 11 side, and the electrons are separated toward the second electrode layer 21 side, and thereby, an electromotive force is generated at the photoelectric conversion layer 31. Accordingly, the first electrode layer 11 toward which the positive holes move is electrically connected to the oxidation electrode 4, and the second electrode layer 21 toward which the electrons move is electrically connected to the reduction electrode 5.

In FIG. 7, the photoelectric conversion layer 31 having a stacked structure of three photoelectric conversion layers is described as an example, but the photoelectric conversion layer 31 is not limited thereto. The photoelectric conversion layer 31 may have a stacked structure of two or four or more photoelectric conversion layers. One photoelectric conversion layer 31 may be used instead of the photoelectric conversion layer 31 in the stacked structure. The photoelectric conversion layer 31 is not limited to the solar cell using the pin junction type semiconductor, but may be a solar cell using a pn junction type semiconductor. The semiconductor layer is not limited to Si, Ge, and for example, may be compound semiconductors such as GaAs, GaInP, AlGaInP, CdTe, CuInGaSe. Note that FIG. 7 is one illustrating an example of the photoelectric conversion element 6, and the photoelectric conversion element 6 is not limited to the silicon solar cell, and other solar cells may be applied. A light irradiation side is not limited to the p-type semiconductor layer, but may be an n-type semiconductor layer.

Operations and the oxidation-reduction reaction of the photoelectrochemical module 1A (104) are described with reference to FIG. 8. Here, sodium hydrogen carbonate (NaHCO₃) is described as a representative example of the intermediate substance, but it is the same in case of the other metal carbonates and metal hydrogen carbonates. When the sunlight and so on are irradiated on the photoelectric conversion element 6, the charge separation occurs by the light energy. The positive holes generated by the charge separation move toward the oxidation electrode 4, and the electrons move toward the reduction electrode 5. The oxidation reaction of H₂O takes place in the vicinity of the oxidation electrode 4 toward which the positive holes move. The positive holes moved toward the oxidation electrode 4 are bonded with electrons generated by the oxidation reaction. The reduction reactions of the intermediate substance and CO₂ take place in the vicinity of the reduction electrode 5 toward which the electrons move. The electrons moved toward the reduction electrode 5 are used for the reduction reaction.

In the vicinity of the oxidation electrode 4, the reaction illustrated in the following (6) expression occurs. H₂O contained in the reaction solution 2 is oxidized (loses electrons), and oxygen (O₂) and hydrogen ions (H⁺) are generated.

2H₂O - - - >4H⁺+O₂+4e ⁻  (6)

In the vicinity of the reduction electrode 5, at first the reactions illustrated in the following (7) expression and (8) expression, further, the reaction illustrated in the following (9) expression occur.

NaHCO₃ - - - >NaOH+CO₂  (7)

2NaHCO₃ - - - >Na₂CO₃+CO₂+H₂O  (8)

2CO₂+4H⁺+4e ^(− - - - >)2CO+2H₂O  (9)

In the vicinity of the reduction electrode 5, CO₂ is generated from, for example, the metal carbonate and the metal hydrogen carbonate as the intermediate substance. CO₂ generated by reactions of the metal carbonate and the metal hydrogen carbonate is reduced (obtains electrons) in the vicinity of the reduction electrode 5. Specifically, CO₂ generated by the reactions of the metal carbonate and the metal hydrogen carbonate reacts with H⁺ which is generated at the oxidation electrode 4 side, defuses in the reaction solution 2, and moves toward the reduction electrode 5 side, and electrons which are generated by the charge separation in the photoelectric conversion element 6 and move toward the reduction electrode 5, and CO and H₂O are generated, for example. Note that the reactions from the (7) expression to the (9) expression are ones illustrating examples of the reactions in the vicinity of the reduction electrode 5, and there is a case when CO and H₂O are generated by direct reduction of the metal carbonate and the metal hydrogen carbonate. Hereinafter, though it is referred to as the reduction reaction of the intermediate substance and CO₂, it indicates any one of the above-stated reactions.

In the vicinity of the reduction electrode 5, it is possible to generate reduction reactions from CO₂ into carbon compounds such as formic acid (HCOOH), methane (CH₄), ethylene (C₂H₄), methanol (CH₃OH), ethanol (C₂H₅OH) in addition to the reduction reaction from CO₂ into CO illustrated in the (9) expression. It is also possible to generate H₂ by generating the reduction reaction of H₂O in the reaction solution 2. It is possible to change a generated reduction substance of CO₂ depending on kinds of the reduction catalyst 9.

The photoelectrochemical module 1B (104) illustrated in FIG. 3 includes the one-liquid type reaction vessel 3 accommodating the reaction solution 2, a photoelectrochemical cell immersed in the reaction solution 2, namely, a stack 10 of an oxidation catalyst layer 12, the oxidation electrode (first electrode) 11, the photoelectric conversion layer 31, the reduction electrode (second electrode) 21, and a reduction catalyst layer 22. According to the photoelectrochemical module 1B, it is possible to enable simplification and so on of components compared to the photoelectrochemical module 1A illustrated in FIG. 2.

The reaction vessel 3 in the photoelectrochemical module 1B (104) has a similar configuration (each pipe and so on) to the reaction vessel 3 of the photoelectrochemical module 1A illustrated in FIG. 2. Note that the photoelectrochemical cell 10 including the photoelectric conversion layer 31 is disposed in the reaction vessel 3, and therefore, the reaction vessel 3 does not chemically react with the reaction solution 2, and is made up of a material which transmits light, namely a material whose absorptance of light in a wavelength region of 250 nm to 1100 nm is low. As formation materials of the reaction vessel 3 as stated above, there can be cited quartz, a super white glass, polystyrene, methacrylate, and so on. The reaction vessel 3 may be one in which only a window part for light irradiation is formed by the above-stated materials, and the other parts are made up of the above-stated resin materials.

As a concrete configuration of the photoelectrochemical cell 10, a structure can be cited in which the oxidation catalyst layer 12 is formed on the first electrode (oxidation electrode) 11 of the photoelectric conversion element 6 illustrated in FIG. 7, and the reduction catalyst layer 22 is formed on the second electrode (reduction electrode) 21. A catalyst layer which is disposed at the light irradiation side between the oxidation catalyst layer 12 and the reduction catalyst layer 22 has the light transmission property. The configuration of the photoelectrochemical cell 10 is not limited thereto, and it is possible to apply the pin junction, the pn junction, the amorphous silicon solar cell, the multijunction solar cell, the single crystal silicon solar cell, the polycrystalline silicon solar cell, the dye sensitized solar cell, the organic thin-film solar cell, and so on having the oxidation catalyst layer and the reduction catalyst layer. It is necessary for the photoelectrochemical cell 10 to create a potential difference which is higher than a difference between a standard oxidation-reduction potential of the oxidation reaction of H₂O generated in the vicinity of the oxidation electrode 11 and a standard oxidation-reduction potential of the reduction reaction of CO₂ generated in the vicinity of the reduction electrode 21. Namely, the photoelectrochemical cell 10 is one capable of providing the energy necessary for simultaneously generating the oxidation reaction of H₂O and the reduction reaction of CO₂

At the photoelectrochemical module 1B illustrated in FIG. 3, it is preferable to constitute such that 50% or more of the light energy being an energy source required for the reaction reaches the photoelectric conversion layer 31 from outside of the reaction vessel 3 by passing through the reaction vessel 3, the reaction solution 2, and the oxidation catalyst layer 12. As composing materials of the oxidation electrode 11 to be at the light irradiation side, there can be cited the metals such as Ag, Au, Al, Cu, the alloy containing at least one of these metals, and the transparent conductive oxides such as ITO, ZnO, FTO, AZO, ATO, and so on as stated above. Here, the oxidation electrode 11 of the photoelectrochemical cell 10 is described to be at the light irradiation side, but it is not limited thereto, and the reduction electrode 21 may be at the light irradiation side. The composing materials of the oxidation catalyst layer 12 and the reduction catalyst layer 22 are as stated above. The catalyst layer which is disposed at the light irradiation side between the oxidation catalyst layer 12 and the reduction catalyst layer 22, further the electrode disposed at the light irradiation side between the oxidation electrode 11 and the reduction electrode 21 have the light transmission property.

The photoelectrochemical cell 10 may be a stack in which the oxidation catalyst layer 12, the oxidation electrode 11, the photoelectric conversion layer 31, the reduction electrode 21, and the reduction catalyst layer 22 are simply integrated, or one in which through holes are formed at the stack as stated above in a stacking direction as ion through holes. It is preferable to provide the through holes at the photoelectrochemical cell 10 to efficiently move H⁺ ions and so on between the oxidation reaction and the reduction reaction, and a reaction efficiency thereby improves. The through holes are provided to penetrate from the oxidation catalyst layer 12 being one surface layer to the reduction catalyst layer 22 being the other surface layer of the photoelectrochemical cell 10. Note that at the photoelectrochemical module 1B illustrated in FIG. 3, the photoelectrochemical cell 10 is immersed in the one-liquid type reaction vessel 3, and therefore, it is excellent in moving efficiency of H⁺ ions and so on by the constitution in itself. It is also the same as the photoelectrochemical module 1A illustrated in FIG. 2. The stirrer and so on may be provided at the reaction vessel 3 to accelerate diffusion of H⁺ ions and so on.

At the photoelectrochemical module 10B (104) illustrated in FIG. 3, the oxidation-reduction reaction similar to the photoelectrochemical module lA illustrated in FIG. 2 occurs as illustrated in FIG. 9. Namely, the sunlight and so on irradiated on the reaction vessel 3 reaches the photoelectrochemical cell 10 via the reaction vessel 3, the reaction solution 2, and so on, then the charge separation occurs by the light energy. The positive holes generated by the charge separation move toward the oxidation electrode 11, and the electrons move toward the reduction electrode 21. The oxidation reaction of H₂O occurs based on the above-stated (6) expression to generate O₂ and H⁺ in the vicinity of the oxidation catalyst layer 12 provided on the oxidation electrode 11 toward which the positive holes move. The reduction reactions of the intermediate substance and CO₂ occur based on the above-stated (7) expression, (8) expression, (9) expression and so on, and the carbon compounds such as CO are generated in the vicinity of the reduction catalyst layer 22 provided on the reduction electrode 21 toward which the electrons move.

Gaseous products containing O₂ generated by the oxidation reaction of H₂O, and the carbon compound (CO and so on) generated by the reduction reaction of the intermediate substance and CO₂ are collected at the upper space S of the reaction vessel 3, and thereafter, transferred to the product collection part 107 via the product send-out pipe 7 e. The generated O₂ and carbon compounds may be supplied to incinerators of, for example, a power station, an ironwork, a chemical factory, a garbage incineration plant, and so on as a carbon fuel containing a combustion improver. It is also possible to separate O₂ and the carbon compounds to be individually used. A part or all of the reaction solution 2 in which the reaction finishes is refluxed to the CO₂ conversion part 102 via the reaction solution lead-out pipe 7 c. The reaction solution 2 refluxed to the CO₂ conversion part 102 is reused at the CO₂ conversion part 102. A part of the reaction solution 2 after the reaction is discharged out of the system via the reaction solution discharge pipe 7 d according to need.

At the phoeoelectrochemical reaction system 100 of the embodiment, CO₂ is converted into at least one intermediate substance selected from the metal carbonate and the metal hydrogen carbonate by the metal hydroxide, and the aqueous solution containing the intermediate substance (a solution containing the intermediate substance and H₂O) is used as the reaction solution instead of absorbing CO₂ by amine molecules which are easy to be oxidized and an expensive ionic liquid. The reaction solution as stated above is used, and thereby, it is possible to apply the one-liquid type reaction vessel 3, and to simplify the electrode structure and the cell structure. The cost reduction, small-sizing, and so on of the device of the CO₂ reduction part 104 become possible. The reaction solution is alkaline, and therefore, it is possible to increase the oxidation reaction efficiency of H₂O. Accordingly, it becomes possible to provide the inexpensive and small-sized photoelectrochemical reaction system 100 which is excellent in the reaction efficiency as a whole of the oxidation-reduction reaction.

Second Embodiment

FIG. 10 is a configuration chart of a photoelectrochemical reaction system according to a second embodiment. A photoelectrochemical reaction system 110 of the second embodiment includes the CO₂ conversion part 102, the reaction solution transfer part 103, a first CO₂ reduction part 104A, a first reaction solution adjustment part 105A, a first reaction solution reflux part 106A, a second CO₂ reduction part 104B, a second reaction solution adjustment part 105B, a second reaction solution reflux part 106B, and the product collection part 107. The photoelectrochemical reaction system 110 of the second embodiment is provided subsidiary to the CO₂ generation part 100X. The photoelectrochemical reaction system 110 may include an impurity removal part 101 in the same way as the first embodiment.

The photoelectrochemical reaction system 110 of the second embodiment includes the CO₂ reduction part 104, reaction solution adjustment part 105, and reaction solution reflux part 106 in two systems relative to the CO₂ generation part 100X, CO₂ conversion part 102, and reaction solution transfer part 103 in one system. The other configurations are the same as the photoelectrochemical reaction system 100 of the first embodiment. Detailed configurations of respective parts 100X, 102, 103, 104, 105, 106, 107 are the same as the photoelectrochemical reaction system 100 of the first embodiment. Though it is not illustrated in FIG. 10, the photoelectrochemical reaction system 110 may include a reaction solution storage part.

One of the system including the CO₂ conversion part 102 and the system including the CO₂ reduction part 104 can be made to be plural systems depending on processing capabilities of the CO₂ conversion part 102 and the CO₂ reduction part 104.

FIG. 10 is an example of a case when the processing capability of the CO₂ conversion part 102 is superior to the processing capability of the CO₂ reduction part 104. In such a case, the system including the CO₂ reduction part 104 is made to be the plural systems relative to the system including the CO₂ conversion part 102, and thereby, it is possible to efficiently operate the CO₂ conversion part 102. This contributes to improvement in the process efficiency as a whole of the photoelectrochemical reaction system 110. It is also the same in a reverse case, and when the processing capability of the CO₂ reduction part 104 is superior to the processing capability of the CO₂ conversion part 102, the system including the CO₂ conversion part 102 can be made to be the plural systems relative to the system including the CO₂ reduction part 104. The other effects are the same as the first embodiment.

Note that the configurations of the first and second embodiments are able to be applied by combination, and a part thereof can be replaced. Here, some of embodiments are described, but these embodiments are presented as examples, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

What is claimed is:
 1. A photoelectrochemical reaction system, comprising: a conversion part converting carbon dioxide into at least one intermediate substance selected from the group consisting of a metal carbonate and a metal hydrogen carbonate by an aqueous solution containing a metal hydroxide, and generating a reaction solution containing the intermediate substance; a transfer part transferring the reaction solution containing the intermediate substance; and a reduction part comprising: a one-liquid reaction vessel where the reaction solution is led in by the transfer part; an oxidation electrode immersed in the reaction solution to oxidize water; a reduction electrode immersed in the reaction solution to reduce the intermediate substance; and a photoelectric conversion element electrically connected to the oxidation electrode and the reduction electrode, and performing a charge separation by light energy.
 2. The system according to claim 1, wherein the conversion part includes the aqueous solution containing a hydroxide of at least one metal selected from the group consisting of an alkaline metal and an alkaline earth metal as the metal hydroxide.
 3. The system according to claim 1, wherein the reduction part includes a stack in which the oxidation electrode, the photoelectric conversion element, and the reduction electrode are integrated, and the stack is immersed in the reaction solution.
 4. The system according to claim 1, wherein the reduction part generates oxygen and hydrogen ions by oxidizing water by the oxidation electrode, and generates a carbon compound by reducing the intermediate substance by the reduction electrode, and wherein a collection part collecting at least one of the oxygen and the gaseous carbon compound generated at the reduction part is provided in the system.
 5. The system according to claim 1, further comprising: a reflux part refluxing the reaction solution where an oxidation-reduction reaction is generated at the reduction part to the conversion part.
 6. The system according to claim 1, further comprising: a storage part temporary storing the reaction solution generated at the conversion part.
 7. The system according to claim 1, further comprising: an adjustment part adjusting at least one of a concentration and pH of the reaction solution which is led into the reduction part.
 8. The system according to claim 1, further comprising: an impurity removal part removing a impurity from at least one of gas containing carbon dioxide which is transferred to the conversion part, and the reaction solution.
 9. The system according to claim 1, wherein the conversion part includes a reaction tank accommodating the aqueous solution containing the metal hydroxide, and a gas supply pipe injecting gas containing carbon dioxide into the aqueous solution accommodated in the reaction tank.
 10. The system according to claim 1, wherein a plurality of the reduction parts are provided in the system, and the transfer part is connected to the plural reduction parts.
 11. The system according to claim 1, wherein pH of the aqueous solution containing the metal hydroxide is 7 or more and 14 or less.
 12. The system according to claim 6, wherein the conversion part operates at night, the reduction part does not operate at night, and the reaction solution generated at the conversion part at night is stored at the storage part.
 13. The system according to claim 4, wherein a gaseous product generated by an oxidation-reduction reaction at the reduction part is collected at an upper space of the reaction vessel, and is then transferred to the collection part from the upper space via a product send-out pipe.
 14. The system according to claim 13, wherein the reaction solution is led into the reaction vessel to be within a range of 50% to 90% relative to an internal capacity of the reaction vessel.
 15. The system according to claim 1, further comprising: a reflux part refluxing the reaction solution where an oxidation-reduction reaction is generated at the reduction part to the conversion part; and an adjustment part adjusting at least one of a concentration and pH of the reaction solution which is led into the reduction part, wherein the reaction vessel includes a reaction solution lead-in pipe connected to the transfer part, an adjustment solution lead-in pipe connected to the adjustment part, a reaction solution lead-out pipe connected to the reflux part, and a reaction solution discharge pipe discharging the reaction solution after reaction.
 16. The system according to claim 15, wherein a lead-in of the reaction solution into the reaction vessel and a lead-out of the reaction solution out of the reaction vessel are performed continuously or discontinuously.
 17. The system according to claim 15, wherein pH of the reaction solution in the reaction vessel is adjusted in a range of 10 to 14 by the reaction solution lead-in pipe, the adjustment solution lead-in pipe, the reaction solution lead-out pipe, and the reaction solution discharge pipe.
 18. The system according to claim 1, wherein the oxidation electrode includes a first support substrate and oxidation catalyst layers formed at both surfaces of the first support substrate, and wherein the reduction electrode includes a second support substrate and reduction catalyst layers formed at both surfaces of the second support substrate.
 19. The system according to claim 3, wherein the stack has a through hole formed in a stacking direction of the oxidation electrode, the photoelectric conversion element, and the reduction electrode.
 20. The system according to claim 10, wherein the transfer part is connected to the plural reduction parts in parallel. 